Many future vehicle safety applications will rely on one-hop Periodic Broadcast Communication (oPBC). The key technology for supporting this communication system is the new standard IEEE 802.11p which employs the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism to resolve channel access competition.
A rapid progress in mobile and wireless technologies in the last decade enables a wide spectrum of applications in the Intelligent Transportation System (ITS) domain targeting vehicle safety, transportation efficiency, and driver comfort. In recent years, many industry/government consortiums are formed around the world to carry out projects to investigate such applications: the Vehicle Safety Communications consortium in the US, the Car2Car Communication consortium in Europe, and the Internet ITS consortium in japan. As a result of these efforts, many interesting vehicle safety application scenarios are identified and their communication requirements are carefully examined. It is now becoming clear that most of these applications will rely on broadcast communication that cones in two flavors: event-driven and time-driven. In the event-driven (or emergency) case, a vehicle starts broadcasting a safety message for a certain duration periodically when a hazardous situation is detected and, hence, these messages are not sent in a normal situation. In the time-driven case, each vehicle continuously performs one-hop periodic broadcast communication (oPBC) to pro-actively deliver a beacon message with its status information (e.g., position, speed) to the neighboring vehicles. The key idea of such oPBC is to make each vehicle aware of its vicinity such that future vehicle safety applications running on the vehicle will leverage this information to detect any hazardous situation in a timely manner. Alone change advisor and a forward collision warning application are two typical examples that rely on this oPBC. These applications require a frequency of 10 messages per second with a maximum no message interval (or a tolerance time window) of [0.3 sec, 1.0 sec]. In addition, these applications pose a strict fairness requirement on oPBC, where each vehicle should have equal opportunity for using the shared channel. In this type of system, message loss is unavoidable (we explain the causes below), however, it must not be the case that one or a few vehicles take all the loss, because this would result in these vehicles becoming a danger to their surrounding vehicles.
We focus on this oPBC from the vehicle safety application perspective. Particularly, we are interested in oPBC which is addressed in the IEEE 802.11p. The 802.11p standard has been designed specifically for inter-vehicle communication. Besides the regular support for higher-layer protocols like IP, the 802.11p Medium Access Control (MAC) supports a short message protocol called WSMP (WAVE Short Messade Protocol, IEEE 1609, where WAVE stands for Wireless Access in Vehicular Environments). Among other uses, this WSMP protocol together with the SAE J2735 addresses the transmission of Basic Security Messages (BSM) also known as beacon messages that are used by a vehicle to inform other vehicles about its status and condition. The BSM (an example of a ‘message’) is sent periodically, in broadcast mode, with a typical frequency of 10 Hz.
In general, the members of the 802.11 family, where the 802.11p is one of the newest members, of wireless standards support two communication modes: a managed mode called Point Coordination Function (PCF) where a base station manages access to the channel and an ad-hoc mode called Distributed Coordination Function (DCF) where stations collaborate to manage channel access. In DCF, stations employ the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism to resolve channel access competition. For point-to-point communication, stations repeatedly perform channel sensing followed by a random Back-off (Bf) period selected from an increasing Contention Window (CW). Bf is used to reduce the probability of a contention problem which occurs when two or more stations that exist in each other's Communication Range (CR) incidentally happen to start transmission at the same time causing collisions. In addition, Request To Send/Clear To Send (RTS/CTS) signaling is used to resolve the hidden terminal, or Hidden Node (HN), problem which occurs when two stations that are outside each other's CR have overlapping transmissions in time interfering with their common neighbors in the intersection of the CRs. On top of this, a MAC level acknowledgement can be used to resolve the remaining message losses. An initial channel access delay, namely Arbitration Inter Frame Space (AIFS), allows discriminating among several priority classes. When stations broadcast messages rather than sending them point-to-point the situation in DCF is quite different. First. CW from which a Bf period is drawn is fixed, and Bf is at most done once. Second. RTS/CTS signaling and MAC layer acknowledgement do not work since there is no particular destination for a message. As a result, when all stations use broadcast-based communication, the collision problems, i.e., the contention and the HN problems increase. FIG. 1a gives an overview of the 802.11p communication behavior in broadcast mode and illustrates the collision problems.
The article “Model, analysis, and improvements for inter-vehicle communication using one-hop periodic broadcasting based on the 802.11p protocol” by Tseesuren Batsuuri, Reinder J. Bill, and Johan Lukkien and the article “Model, analysis, and improvements for vehicle-to-vehicle communication using one-hop periodic broadcasting based on the 802.11p protocol” by the same authors and filed as the priority text of this application are both incorporated by reference.